R51/3 one carb will not idle down

[jimstravlin]

I am finishing up a 1953 R51/3 and I can not get the left hand carb to idle down. The slide is on the bottom of the carb body, I have checked and re-checked all internal parts which are all new. The body has been cleaned in carb cleaner 2 times, plenty of free play in the cable, all internal passages have been blown clear. Any ideas, anyone?

[schrader7032]

So when you look into the carb throats with the air tubes off and engine running, are the slides in the same position? Where are the settings for idle mixture (screw that controls the amount of air let in at idle) and the idle stop screw?

[jimstravlin]

Idle mix screw is at the bottom with no pressure on the slide, slides sitting on the bottom of the carb body

[schrader7032]

Generally, a starting point for those slide screws is one turn in after making contact with the bottom of the slide. As part of a carb synch, you would adjust those screws so that you had equal RPM on each cylinder. If the slides are sitting on the bottom of the carb body, that seems like it's running too rich...high velocity air is getting by the sliding and sucking up fuel from the bowl.

[Beemer100]

Check Pic 114 in the manual:

https://cms.bmw-einzylinder.de/images/P ... 9S_WHB.pdf

You need to have 2mm slack between the inner and the outer cable.

[palica]

Did you check if you have air leak (with a can of WD40 or just a little brush with gas, applied on the carb intake and body).
A poor mix (more air than gas) will increase idle, so this could be the problem.
Carb flange can warp as well, which causes air leaks.

https://w6rec.com/fix-intake-leaks/

[jimstravlin]

Thanks all... I have tried all of these suggestions, still looking.

[Beemer100]

now, that sounds really interesting.

I think, in order to go to the next step, we would need to know exactly what you have done and how the issue happens.

The numbers in brackets are referring to the pics at bmwbike.com table 13_0829 subtable 13.09 (carburetors for Boxer)

Check both cables to the carbs, if they are wired smoothly from the throttle grip to the carb and there is no sharp curve whatsoever. Check also the spring (6) if it has enough strength to snap back the throttle grip once you let it go.

Then, do as Kurt already said: Important is to do standard adjustment to the carb. For the first step I use very small cable ties on both sides and fiddle them between the carb slide and the lower part of the body. Turn out the slide stop screw (13 adjuster) so that the slides sits on the body. Then slowly turn it back in, until the cable tie starts to tremble. One half turn further in, should be a good start. You do this on both sides. Then (cable ties back under both slides) you loosen the counter nuts (4 nut) on the carb bodies cap in order to adjust the adjuster (3). Fix one first, so that the cable play at the adjuster (3) is roughly 2mm. Then adjust the second one so that once you turn the throttle, you see both cable ties tremble at the same time. Once that is done, make sure that you still have enough cable play also on the recently adjusted side (roughly 2mm). All of that can be done without even starting the engine and it will give you a good start for your synchronization of the carbs later once warmed up.

Now, you say, all of that is done but still one side doesn't idle properly. Test both cylinders separately (make sure you put a spark plug into the pulled socket and lay it on the cylinder so that it's grounded). Start the bike on the active cylinder and see if it reacts to the throttle like it should. Also check if the engine reacts to a change of the air mix screw (16). Test both cylinders separately.

report back your findings.

good luck
Klaus

[Jim D 5112]

I would consider just removing the throttle cable from the side that won't idle down after it is warmed up. Then just see if it will idle down. That will eliminate the whole cable thing. Then you will know if it is the carb intake area. You could even make a short cable with a loop on it to throttle the carb right their by it.

[MikeL46]

I would try swapping the carbs or just the carb tops to see if the problem follows either one.

Mike